1. Field of the Invention
The present invention relates to bleaching of wood pulps, and in particular to peroxide bleaching of mechanical wood pulps using magnesium hydroxide as a source of alkali.
2. Prior Art
Mechanical pulps are produced from wood using mechanical means only. There are four mechanical pulping processes in use commercially: the stone groundwood (SGW), pressurized stone groundwood (PSGW), refiner mechanical (RMP) and thermomechanical pulp (TMP) pulping processes. The stone groundwood process and the pressurized stone groundwood process use wood bolts while the refiner mechanical and thermomechanical pulping processes use chips. The stone groundwood process is the leading process for mechanical pulping but is rapidly being replaced by the thermomechanical pulping process because there are distinct economies that arise from using chips rather than wood bolts, and the resultant thermomechanical pulp is inherently stronger. In mechanical pulping, no active chemical, other than water, is used to facilitate fiber liberation. Virtually, all of the chemical constituents in the wood are retained in mechanical pulp, including lignin and other chromophores, which cause darkening of the pulp.
Mechanical pulps are typically bleached to enhance their brightness. In the bleaching process, hydrogen peroxide and caustic soda (i.e. sodium hydroxide) are widely used bleaching chemicals. Hydrogen peroxide forms perhydroxyl anions, which are the active bleaching agents. An alkali, such as sodium hydroxide, must be present to provide the hydroxyl anion necessary to generate the perhydroxyl anion. The perhydroxyl anions that are formed release oxygen for slow bleaching of mechanical pulps without degrading the cellulose polymer of the pulp. If hydroxyl radicals are formed instead of perhydroxyl anions, damage to the cellulose polymer will occur and a decrease in pulp strength will result.
Auxiliary chemicals such as caustic soda, sodium silicate, and DTPA (diethylenetriaminepentaacetic acid) or EDTA (ethylenediaminetetraacetic acid) are typically added along with hydrogen peroxide to create a stabilized environment for the formation of perhydroxyl anions. Caustic soda is a strong base that provides the alkalinity and pH (9–11) that are generally thought to be necessary to promote the bleaching process. However, caustic soda can be harsh to pulp fibers. Sodium silicate is typically used as a stabilizer in conjunction with DTPA, a chelant, to prevent catalytic destabilization of hydrogen peroxide into harmful radicals. Both sodium silicate and DTPA are believed to scavenge transition metals such iron, manganese, and copper, which catalyze the decomposition of hydrogen peroxide. Sodium silicate has a disadvantage in that it has a tendency to scale and is abrasive in refiner bleaching.
In peroxide bleaching of mechanical pulp, the main objective is to raise the brightness (whiteness) of the pulp without sacrificing pulp yield. The lignin-carbohydrate matrix should be maintained without dissolving any solid substance other than the extractive components in the wood. Pulp yield is critical because the cost of the raw wood represents a significant portion of the manufacturing cost of pulp.
The mechanical pulp industry uses primarily two bleaching agents: sodium hydrosulfite (sodium dithionite Na2S2O4), a reducing agent, and hydrogen peroxide, an oxidative bleaching agent.
Hydrogen peroxide can react with chromophoric groups or sites on the lignin polymer, usually conjugated carbonyl groups that have a propensity for absorbing visible light. Hydrogen peroxide can partially destroy these chromophoric groups, thus raising the brightness or whiteness of the pulp. The ISO brightness scale ranges from 0%, which is a black body, to perfect whiteness of 100%, given by a MgO standard crystal. Depending upon the processing conditions and the age of the wood, unbleached TMP pulp typically has a brightness between 55 to 60 on the ISO scale, compared to unbleached stone groundwood pulp, which is 60 to 65. For mechanical pulp such as TMP, the brightness gain using hydrogen peroxide is typically 10 to 15 brightness units using the ISO brightness scale.
In hydrogen peroxide bleaching, the perhydroxyl anion, OOH− is generally regarded as the active species that does the bleaching. The perhydroxyl anion occurs through dissociation:H2O2=OOH−+H+  (1)The dissociation is strongly affected by pH and to a lesser extent by temperature.
The addition of an alkali and the control of the bleaching temperature can regulate the concentration of the perhydroxyl ion. Adding alkali shifts the equilibrium to the right and raises the concentration of the perhydroxyl anion according to the following equation:H2O2+OH−=OOH−+H2O  (2).In bleaching mechanical pulps, the pH is typically maintained in the range between 10.8 to 11.2 with the aid of a buffer, such as sodium silicate, to avoid excess peroxide decomposition. Typical levels of caustic addition range from about 1% to 3%, (wt % based upon the pulp mass), and depending upon the alkalinity of the system.
Decomposition of hydrogen peroxide to forms other than the perhydroxyl anion is to be avoided because hydrogen peroxide is expensive. If the peroxide decomposes to forms other than the perhydroxyl ion, then less perhydroxyl anion is available for bleaching. Hydrogen peroxide dissociates into various free radical species according to the following equations:H2O2=OH−+OH+  (3)H2O2=OOH•+H•  (4)H2O2=OH•+OH•  (5).The hydroxide free radical (OH•) is thought to decompose the carbohydrate components, cellulose and hemicellulose polymers, found in the wood. This is an important consideration when bleaching chemical pulps, but is not a major consideration when bleaching mechanical pulps. Mechanical pulps are added primarily as a filler, but not for strength, and must provide opacity, brightness, and print quality.
Hydrogen peroxide can further decompose to form oxygen through the following reaction with the perhydroxyl ion:H2O2+OOH−=OH−+O2(g)+H2O  (6).The maximum amount of decomposition occurs at 50% dissociation or when the pH is equal to the pK for the dissociation reaction. Approximately 10% of the available hydrogen peroxide decomposes to perhydroxyl anion OOH− at pH 10.5. The decomposition increases to approximately 95% of the available hydrogen peroxide at pH 12.5. The pH is controlled to a value of 10.8 to 11.2 when bleaching mechanical pulps to control the decomposition of both the perhydroxyl anion and the unreacted peroxide.
The decomposition of hydrogen peroxide is catalyzed by the presence of metal ions, notably manganese, iron, and other transition metals. Overall, the process can be represented by the following equations:2H2O2+M+2=M+3+OH−+OH•  (8)H2O2+M+3=M+2+H•+OOH•  (9)OOH•=O2(g)+H•  (10)H•+OH•=H2O  (11).These decomposition reactions remove peroxide before it can dissociate to form the perhydroxyl anion (given by equation (2)) and participate in bleaching reactions. Metals removal and control of the bleach liquor is an important part of the efficient use of hydrogen peroxide bleaching of mechanical pulps.
Mechanical pulps are typically pretreated. The purpose of pretreating mechanical pulps prior to hydrogen peroxide bleaching is to tie up and wash out most of the transition metals present in the pulp prior to the addition of the bleaching liquor. Metals originate from the wood and the piping system, and both sources must be controlled. There are two principle methods used commercially to manage the metals in peroxide bleaching: (1) by stabilization of the mixture with sodium silicate, and (2) pretreatment and subsequent removal of metals from the pulp with an organic chelating agent.
Adding sodium silicate to the bleach liquor is thought to have two benefits: it significantly reduces peroxide decomposition and it improves the stability of the bleach liquor. Approximately 3% sodium silicate (wt %, based on pulp mass) is added when bleaching mechanical pulps where scaling is not a problem. In cases where scaling is severe, that is, when closed water loops allow buildup, a typical dose rate is 1% to 2%. When bleaching is done in refiners, sodium silicate is avoided to minimize scale buildup and reduced refiner plate life from abrasion. When sodium silicate cannot be used, organophosphonates are sometimes employed. The use of organic stabilizers is not commonly practiced because of poor performance and unfavorable economics.
The exact mechanism by which sodium silicate functions is not precisely known. Sodium silicate is thought to act as a metal ion sequestrant, as a buffering agent, and as a promoter of metal surface passivity. With regard to stabilization, metal sequestration and rendering metal surfaces passive are two important functions of sodium silicate. However, one major disadvantage of sodium silicate is scaling. As a result, there is a need for an inorganic substitute for sodium silicate.
A second common method for metals control involves pulp pretreatment using an organic chelating or sequestering agent. The material must be compatible with the hydrogen peroxide and must also be able to form a complex with the metallic ions. Typically, the pentasodium salt of diethylenetriaminepentaacetic acid (Na5DTPA) is used in this role. The pretreatment is usually carried out at low consistency (consistency being wt % pulp in the pulp-liquor mixture), typically 3–5%, in a latency chest following refining, at a pH of 4 to 6.0. The pulp is then thickened prior to bleaching to moderate or medium consistency (6% to 14%) using a decker (6% to 8%) or disk filter (10% to 14%), or to a high consistency (20% to 25%) using a belt-press or a twin-roll press. This thickening step is very important as the complexed metals are washed from the pulp during the change in solids level. If the thickening step prior to bleaching is not possible, the treatment will still work, but will be less effective. The bleach liquor that is applied to the mechanical pulp to bleach mechanical pulp is a mixture of caustic soda and hydrogen peroxide in water. The bleaching liquor may also have other components to aid the bleaching reactions. Most often it contains some level of sodium silicate (41° Be), usually 1% to 3%, measured on an oven dry basis. Sometimes the bleach liquor will contain magnesium sulfate if it has been determined that extra Mg+2 ion will aid in liquor stability, and therefore the overall brightness gain of the pulp. Table 1 gives the composition of typical liquor for bleaching mechanical pulp.
TABLE 1Typical Peroxide Bleaching Liquor Amount Added Based Upon PulpComponentAmountHydrogen Peroxide (H2O2)0.5% to 4%  Caustic Soda (NaOH), 100%1.0% to 2.5%Sodium Silicate(a), 41° Be2% to 5%DTPA0.15% to 0.3% (a)Water glass with a typical ratio Na2O · 3.75SiO2
In a typical hydrogen peroxide bleaching process, wood pulp is combined with caustic soda (NaOH), sodium silicate and a chelating/sequestering agent.
The typical hydrogen peroxide bleaching process according to the prior art is characterized by the following problems:
High pH, which must be adjusted prior to discharge of effluent to outfall pipes.
Sodium silicate scaling, which reduces its utility.
Use of caustic soda, which is a strong base, and which tends to degrade wood pulp resulting in relatively low pulp yields and high chemical oxygen demand (COD.)
High concentrations of caustic soda and sodium silicate result in high concentrations of anionic “trash” in bleaching effluent, which in turn requires the use of retention chemicals in later paper-making processes.
While it has been known to use magnesium salts, such as magnesium sulfate (MgSO4), magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2), in the hydrogen peroxide bleaching of mechanical pulps, it has heretofore been believed that peroxide bleaching must be carried out at elevated pH, e.g. between 10 and 12, in order to ensure sufficient concentration of hydroperoxyl anion (HOO−) to oxidatively destroy chromophoric groups in the wood pulps. Accordingly, it has been known in the art to partially replace sodium silicate or sodium hydroxide with magnesium salts. However, it has not heretofore been known to conduct hydrogen peroxide bleaching of wood pulps at or near neutral pH 5.0–8.5 in the presence of magnesium hydroxide as the alkali source.
There is, therefore, a need for an improved method of bleaching wood pulps with peroxide that does not use sodium silicate or added caustic (e.g., NaOH). There is also a need for a method of bleaching wood pulps that can be performed at neutral pH (e.g., 5.0–8.5), and that produces less COD and anionic trash than prior art methods. There is furthermore a need for a process of bleaching wood pulp that permits economical recycling of unused hydrogen peroxide. There is also a need for a method of bleaching wood pulp that produces brightness values of greater than about 71%, and up to about 75% while excluding added silicate in most cases and/or caustic.